Respiration, the consumption of dioxygen at the cellular level to provide energy for metabolic processes, is mediated by terminal oxidases such as cytochrome c oxidase (CcO). Recent experimental work has revealed more complexity in the regulation of energy production by CcO than hitherto appreciated. One specific factor has been the realization that nitric oxide (NO), a gaseous free radical, has an important role in vivo as a competitive inhibitor of dioxygen binding to CcO. Aside from the normal, although poorly understood, regulatory role of NO in vivo this important discovery opened the prospect that a considerable number of pathological conditions arise from abnormal levels of NO and its subsequent reactions with proteins such as those of the electron transport chain (of which CcO is terminal element). Parkinson's, Alzheimer's, Huntington's disease and ALS have been linked to the NO/CcO couple. While there are a plethora of phenomenological experiments demonstrating the connection between energy regulation and NO levels, the mechanism of reaction of NO and the physicochemical characteristics of intermediates at the heme/copper active site of CcO as they relate to regulation and pathology remain controversial. Significantly, an ancestral progenitor of CcO, Nitric Oxide Reductase (NOR), is not inhibited by NO, but reduces it to N20 with release of energy, a reaction analogous to the reduction of O2 to H2O by CcO. The mechanism by which NOR reduces NO and the relationship of its structure to this activity is little known. Continuing a long-term goal of understanding biomimetic reactions of small molecules at the active sites of terminal oxidases, this project aims to characterize the reaction of NO with heme/copper (CcO) and heme/iron (NOR) biomimetic complexes. Through spectroscopic and electrochemical investigation of previously-developed ligand systems, using numerous bimetallic complexes with metal ion combinations not possible to obtain biologically, but essential for isolating the factors that are contributing to the enzymatic characteristics in vivo, we aim to answer many questions concerning the differences in NO reaction with CcO and NOR. The identification of reaction intermediates, their reactivity, leakage and possible biological consequences will be a focus for study. An emphasis will be placed on kinetic and mechanistic studies to elucidate the putitive reaction pathways of CcO and NOR.